Coal ash is the generic term referring to several distinct materials produced when coal is combusted to produce electricity. The term “coal ash” may include, but not necessarily be limited to, fly ash, bottom ash, boiler slag, and various forms of flue gas emission control or desulfurization materials. Each of these types vary by coal source and composition, the combustion technology used, the emissions control technology used, and other factors. It is better to find a use for coal ash instead of disposing of it in landfills, thereby avoiding the environmental degradation and energy costs associated with mining unused materials. For instance, for every ton of fly ash used in place of Portland cement, about a ton of carbon dioxide is prevented from entering Earth's atmosphere. A significant benefit of using fly ash is that it requires less water than Portland cement, conserving a limited resource, while also reducing a project's water and equipment costs. Boiler slag, which replaces sand in blasting grit, has the benefit of being free of silica, which eliminates the potential health risk of silicosis. Flue gas desulfurization materials are used in 30% of US wallboard products, avoiding the need to mine gypsum. Environmentally and economically it makes more sense to use existing materials than to mine new ones.
It is more convenient and economical to transport coal ash as a dispersion or slurry in water, where the coal ash dispersion may be pumped and transported through pipes. However, coal ash dispersions have two disadvantages. First, if the coal ash is not dispersed in the water medium in a stable way, it may settle out and cause fouling of the pumps, pipes, and other transportation apparatus used to move it from one place to another. Second, coal ash dispersions have a tendency to cause calcium carbonate (CaCO3) scaling of the equipment it passes through, including pumps, pipes, tanks, etc. Scale deposits may also cause fouling if they are severe enough.
U.S. Pat. No. 4,441,887 discloses a stable slurry containing carbonaceous solid material (such as coal or coke), a carrier liquid (such as water or oil), and a dispersing agent. Also disclosed is a method of preparing the slurry by grinding a carbonaceous solid material at a high solids content of at least 60 volume percent to generate at least 5 weight percent of colloidal carbonaceous particles and mixing the carbonaceous solid material so ground with carrier liquid and dispersing agent until a slurry with specified properties is produced. A method of stabilizing a carbonaceous solid material-liquid slurry by adding to the slurry either fine carbonaceous solid material and/or dispersing agent and/or stabilizer is also disclosed.
U.S. Pat. No. 4,623,359 discloses aqueous slurries of solid carbonaceous fuel that include a novel highly converted sulfonated surfactant derived from a suspension of low rank coal and coal derived creosote oil for reducing the viscosity and increasing the solids content of the aqueous slurry.
It would be desirable if new methods and compositions were found which permitted the simultaneous prevention or inhibition of both fouling and scaling of coal ash dispersions.